Rotating biological contactors (RBC's) are used for the biological treatment of wastewater. Such a device or system characteristically utilizes at least one wastewater contacting medium which is fixed upon a rotatable shaft that is arranged to continuously revolve in a reservoir of wastewater to be treated. As the contacting medium revolves, a biological culture naturally develops on it and this culture has the capacity for digestion of contaminating substances in the wastewater. As the contacting medium bearing the culture rotates, it experiences alternatively exposure to wastewater and then to oxygen (air), thereby achieving aerobic treatment of the water contaminants. Alternatively, the medium can be completely submerged within the wastewater for anoxic and/or anaerobic processes to treat the waste stream.
Currently, what is believed to be the most common commercially available form for an RBC utilizes a single, horizontally rotating shaft about eight meters (about 25 feet) long which carries contacting media or contactors having a gross diameter of about four meters (about 12 feet) which provide a total of about 10,000 square meters (about 100,000 square feet) of media surface area. Commonly, such one shaft extends across a single reservoir or stage. Multiple reservoirs (or multiple stages) can be used which are preferably arranged so as to be successively coaxial or longitudinally adjacent to achieve a maximized wastewater treatment. In RBCs having a longitudinal axis, the tank or trough (usually hemi-cylindrically sided) may be subdivided into a plurality of axially aligned, longitudinally adjacent stages by means of transversely extending baffles or bulkheads. Such a multistaged-type of RBC is commercially available in various sizes from various manufacturers. In a commercially available RBC, circular contactors may be employed which have contactor diameters ranging from about 0.7 meters (about 2 feet) to about 4 meters (about 12 feet), and the number of individual stages ranges upwards from one.
It is most often desirable to design an RBC with multiple stages, generally three or more. Experience teaches that by providing the requisite amount of media (contactor) surface area for bacterial colonization into multiple, sequential stages, performance is enhanced. This sequencing of stages promotes a tendency for bacterial specialization to occur along the sequence as specific contaminants are destroyed by those bacterial species which have the fastest rate of consumption. This is comparable, for example, to making an activated sludge reactor very long or a packing tower (trickling filter) very tall. If the same media surface area were divided among stages in parallel, even though the total flow rate through and the total volume of the sum of all stages (and hence the waste water contact time per unit area of media surface area) remain then same, the advantage of sequential processing (and bacterial specialization) would be missed.
Commonly, the trough or tankage portion of a conventional large commercially available RBC is typically locally designed and built to accommodate rotating circular contactor(s) that are supported on an associated shaft and shaft drive assembly purchased from the manufacturer. Such a tankage portion is commonly fabricated on-site of poured concrete. Smaller, test RBC units are available that employ steel tankage. Also, still smaller units suitable for residual wastewater treatment, and for the aquaculture and aquarium industries, are available which have tankage portions comprised of fiberglass or other plastic. However, in all known commercial forms of RBC's, the tankage portions are individually designed and fabricated either to order or as packaged assemblies, and all such RBC embodiments are either fixed, or are characterized by having very limited variability in operational configuration. Their stationary components, such as the tankage portion, are dimensionally fixed.
For an RBC installation to treat wastewater, it is desirable initially to evaluate and characterize the particular wastewater and to design and adapt a RBC system for usage with that particular wastewater so that system parameters are optimized for best or maximized effective treatment of that wastewater. Various RBC design procedures and criteria are known to those skilled in the art. If desired, for design purposes, a miniaturized RBC embodiment may be preliminarily used to evaluate samples of the wastewater and identify operating parameters. For example, one suitable miniaturized RBC system is disclosed in my U.S. Pat. No. 4,737,278.
It has become routine for a wastewater to be characterized and for mathematical models to be used to design RBC installations for treating that wastewater; see, for example, “Design of Biological Treatment Systems,” pages 25–66–25–76 in Perry's Chemical Engineers' Handbook, Seventh Edition (1997) McGraw Hill. However, there are wide variations in wastewater characteristics.
Because of the many variations in application situations, a need exists for RBC apparatus which can be rapidly, simply, reliably and economically fabricated and assembled, and then later, if need be, modified and/or expanded even after an initial installation has been completed and operated. A plurality of various potential RBC apparatus configurations would be desirable using the same components. The capability for using the same components to construct a variety of RBC assemblies would, if available, offer many practical advantages, especially where the characteristics of a particular wastewater are incorrectly initially determined, or substantially change over time. To this end, the technology of my U.S. Pat. No. 4,729,828 was provided to introduce modularity into RBC design considerations.
Though the technology of my '828 patent is very useful, it would be desirable to improve such. For one thing, it would be desirable to improve system versatility so that an assembled RBC comprised of preformed components can have a greater variety of different configurations. In the '828 system, for example, the tank or trough volume of each successive stage is determined by the interior length of a preformed tank-defining section which has fastened at and across each longitudinal opposite end thereof a bulkhead-type member. For greater versatility, the interior length of the trough of a single stage needs to be variable incrementally. Selected tank housing sections need to be joinable adjacently without the need for a bulkhead-type member positioned between each pair of longitudinally adjacent tank housing sections. Outside support side walls of trough defining sections need to be separate from, but associatable with, the trough defining sections themselves in certain circumstances.
For another thing, the respective longitudinally extending individual shaft structures when located in each stage should be more readily length adjustable and should be more readily, simply and reliably connectable with, and disconnectable from end adjacent, coaxial end shafts of shaft structures located in longitudinally adjacent stages so that all shafts, as coaxially interconnected, rotate together yet be readily connectable and disconnectable.
For another thing, it would be desirable for the assembled RBC to be operationally more energy efficient so that less applied power, particularly electric power, would be needed in system operation for shaft rotation. In, for example, some RBC installations of the type taught by my '828 patent, relatively large (and thus relatively expensive) amounts of electric power may need to be expended in rotating the shaft.
For another thing, it would be desirable for an RBC to be fittable into an existing relatively confined structure (such as a building, pit, etc.) and to be readily assembled from prefabricated components, even by only two men using simple tools.
The developing field of aquaculture brings new and additional challenges to wastewater treatment and to RBC structures useful in treating wastewaters from such field. A desire for recirculated water and water reuse for use in aquaculture production has developed which necessitates solving the problems of achieving consistent water quality while conserving the use of water and the energy required to maintain stable water temperatures for continuous production at latitudes where substantial environmental temperature variation is normal. Also, aquaculture facilities that employ more traditional fish production techniques employing only new water and require large wastewater flows may no longer be appropriate. This is especially true if the aquaculture facility is located near desired markets where competing uses of land and water limit access to resources and require constraining demands on wastewater.
Each species of fish cultured requires an optimized water temperature for rapid growth. This temperature typically falls in the range from about 5° C. to about 40° C. Likewise, salinity influences fish culture and typically falls in the range from 0 to about 40 ppt. Also, unique dietary requirements for these cultured species can include protein contents typically ranging from less than about 30% to greater than about 50%. Dietary protein ultimately leads to ammonia contamination of the culture water. Typically, those fish species which require a lower temperature environment also require the highest dietary protein. When using a biological filter, such as an RBC, both these conditions require a relatively large contactor media surface area to assure sufficient biological activity for purposes of achieving and maintaining a set of desired or necessary water quality conditions, particularly when water recirculation is contemplated. As temperatures fall, bacterial activities slow, thus requiring increased medium surface area to obtain sufficient bacterial colonization to obtain desired levels of nutrient destruction. Likewise, as the protein level in the feed increases, ammonia production increases, requiring increased medium surface area to obtain the requisite ammonia destruction.
With relatively stringent water quality parameters, such as total ammonia nitrogen (TAN) as low as 1.0 mg/l and nitrite nitrogen (NO2–N) as low as 0.1 mg/l on the outlet from the wastewater treatment unit, RBC design specifications which will result in such output water parameters are little understood. RBC equipment and operating conditions have heretofore commonly not been commercially available to readily satisfy such stringent and/or variable circumstances. To permit or enhance the use of RBCs in such demanding conditions and circumstances, it would be desirable to have convenient and readily assembled or disassembled improved components for use in RBC systems.
Particularly in aquaculture, the need is great for relatively low cost, economically operating RBC's that are comprised of relatively low cost, easily transported, handled and assembled components and that can achieve output water which meets stringent water quality parameters and so is recirculatable. A large user component of the aquaculture field is comprised of small entrepreneurial and family farm establishments that utilize existing structures as aquaculture facilities, such as garages, barns, idle livestock production buildings, warehouses, and the like. An embodiment of RBC apparatus manufactured by the prior art techniques and components, such as above indicated, proves extremely difficult to fabricate and use from the standpoints of cost, transportation, and user assembly in such small facilities. The problem is exacerbated by the fact that many of these facilities have a ceiling height of less than about three meters (about 10 feet). RBC components need to be small enough for convenient transport and for assembly in such a structure by no more than two individuals.
It appears that, particularly in the aquaculture field, readily assemblable RBC components and subcomponents that are low cost, adaptable for use in small space, and easily manipulated are needed and would potentially enjoy wide usage.